Efficient industrial processes require the measurement and control of various of parameters such as temperatures, pressures, levels and flow rate. Instruments are used to effect those measurements. A typical instrument has three components; a sensing element; a transmitting element and an output or indicating element.
Sensing elements respond directly to the measured quantity, producing a response such as displacement or motion, pressure, or electrical signal. The response is transmitted by a transmitting element which may include linkages, tubing, wiring that provides an output signal. The transmitting elements may comprise one or more transducers. Among the types of transducers that are commonly used are mechanical transducers that convert one form of energy into other form that can be measured easily. For example, a linear force may be converted to torsion, and torsion may be converted to displacement. The output signal of the transmitting element is received by the output or indicating element that displays a representation of the output signal. For example, displays may include, among other devices, a dial with a needle indicator, or digital displays.
A level transmitter or controller (e.g. Dresser Masoneilan 12400 Series Digital Level Transmitter/Controller) is an example of such an instrument. Level transmitters are used to measure the level of a liquid in a reservoir or vessel and may be incorporated in control systems that control industrial processes in a variety of industries. Level transmitters or controllers may incorporate two-wire field devices coupled to a control room using a two-wire process control loop. Two-wire devices receive power from a process control loop, and communicate over the process control loop. Some instruments use Highway Addressable Remote Transducer (HART®) protocol for sending and receiving digital information across analog wires between the instrument and control or monitoring systems. An exemplary level transmitter or controller may comprise a displacer (sensing element) that is immersed in the liquid. The displacer is coupled to a torque tube. A change in liquid level varies the net weight of the displacer, increasing or decreasing a torsion load on a torque tube and torque rod by an amount directly proportional to the change in liquid level. The torque rod is attached to a magnet assembly comprising a rotating beam with an attached magnet. The rotation of the torque rod results in the angular displacement of the magnet assembly. The displacement of the magnet modifies the magnetic field surrounding a non-contact sensor, producing a signal proportional to the level in the vessel. The signal may be provided to an output readout component that provides a measure of the level of the liquid. These instruments are rugged, reliable and accurate. [hart is example any communication works]
In rare situations the mechanical linkage between the torque rod and the rotating beam may be disconnected. When that happens, it is possible for the beam and magnet to be in a position where the non-contact sensor provides an erroneous but plausible output. This possibility creates a problem when the instrument is used as a component of a safety instrumented systems (SIS) that is used to achieve or maintain a safe state of a process when unacceptable or dangerous process conditions are detected. The consequences of an erroneous but plausible process condition may be severe. The quality or dependability of an SIS is conveyed by safety integrity levels ratings (SILs). There are four discrete integrity levels associated with SILs. The higher the SIL level, the lower the probability of failure on demand for the safety system and the better the system performance. In some cases, an instrument having a mechanical connection may not be SIL 2 rated unless the instrument provides a way to detect a broken mechanical connection. For example, in the case of Dresser Masoneilan 12400 Series Digital Level Transmitter/Controller, the instrument may not be SIL 2 rated unless there is a way to detect when the magnet assembly is disconnected from the torque rod.